Chapter 5: Problem 320
Use the ratio test to determine whether \(\sum_{n=1}^{\infty} a_{n}\) converges, where \(a_{n}\) is given in the following problems. State if the ratio test is inconclusive. $$a_{n}=n^{10} / 2^{n}$$
Short Answer
Expert verified
The series converges.
Step by step solution
01
Understand the Ratio Test
The ratio test for convergence states: given a series \( \sum_{n=1}^{\infty} a_n \), compute \( L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| \). If \( L < 1 \), the series converges; if \( L > 1 \) (or \( L \) is infinite), the series diverges; if \( L = 1 \), the test is inconclusive.
02
Identify and Simplify \( a_{n+1} \)
Given \( a_n = \frac{n^{10}}{2^n} \), calculate \( a_{n+1} = \frac{(n+1)^{10}}{2^{n+1}} \). This simplifies to \( \frac{(n+1)^{10}}{2^n \cdot 2} \).
03
Formulate the Ratio \( \frac{a_{n+1}}{a_n} \)
Calculate \( \left| \frac{a_{n+1}}{a_n} \right| = \frac{\frac{(n+1)^{10}}{2^n \cdot 2}}{\frac{n^{10}}{2^n}} = \frac{(n+1)^{10}}{2 \cdot n^{10}} \). Simplifying gives \( \frac{(n+1)^{10}}{2n^{10}} \).
04
Compute the Limit
Evaluate the limit \( L = \lim_{n \to \infty} \frac{(n+1)^{10}}{2n^{10}} \). This can be rewritten as \( \lim_{n \to \infty} \frac{(n^10 + 10n^9 + ... + 1)}{2n^{10}} \). The dominant term here is \( \frac{n^{10}}{2n^{10}} = \frac{1}{2} \). Thus, \( L = \frac{1}{2} \).
05
Apply the Ratio Test Conclusion
Since \( L = \frac{1}{2} < 1 \), the ratio test indicates that the series \( \sum_{n=1}^{\infty} a_n \) converges.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Convergence of Series
In the world of mathematics, determining whether a series converges or diverges is an important skill. A series involves adding up an infinite number of terms. The Ratio Test is a popular method used to determine the convergence of a series. By using this test, we can efficiently decide if the series leads to a finite sum or continues indefinitely without settling.
The key steps involve computing a limit, denoted as \( L \), for the absolute value of the ratio of subsequent terms of the series. Specifically, if \( L < 1 \), the series converges, meaning it sums up to a finite number. This gives us assurance about the behavior of the series.
However, if \( L > 1 \) or \( L \) is infinite, the series diverges, meaning that it grows indefinitely and does not sum up to a finite number. When \( L = 1 \), the test is inconclusive, and other methods or tests need to be applied to check for convergence.
The key steps involve computing a limit, denoted as \( L \), for the absolute value of the ratio of subsequent terms of the series. Specifically, if \( L < 1 \), the series converges, meaning it sums up to a finite number. This gives us assurance about the behavior of the series.
However, if \( L > 1 \) or \( L \) is infinite, the series diverges, meaning that it grows indefinitely and does not sum up to a finite number. When \( L = 1 \), the test is inconclusive, and other methods or tests need to be applied to check for convergence.
Limit of a Sequence
The limit of a sequence is the value that the terms of a sequence approach as the index (usually denoted as \( n \)) goes to infinity. It's an essential concept that helps us understand the behavior of sequences in mathematical analysis.
When applying the Ratio Test to a series, knowing the limit of a sequence is crucial because it determines the outcome of the test. Calculating the limit involves simplification and finding the leading behavior of terms in a sequence. For example, if the sequence corresponds to a term like \( \lim_{n \to \infty} \frac{(n+1)^{10}}{2n^{10}} \), simplifying the expression finds the dominant factor.
When applying the Ratio Test to a series, knowing the limit of a sequence is crucial because it determines the outcome of the test. Calculating the limit involves simplification and finding the leading behavior of terms in a sequence. For example, if the sequence corresponds to a term like \( \lim_{n \to \infty} \frac{(n+1)^{10}}{2n^{10}} \), simplifying the expression finds the dominant factor.
- Identify the dominant term within the fraction to simplify calculations.
- Evaluate the limit as \( n \) grows, usually simplifying to a fraction of constants.
Infinite Series
An infinite series is the sum of infinitely many terms, which are often derived from a sequence. Exploring infinite series provides insight into how these endless summations behave, and whether they can sum to a finite value.
In calculus, infinite series are vital because they often represent complex functions in a simplified manner via power and Taylor series. To analyze these series, tests like the Ratio Test are applied. The convergence of an infinite series tells us whether the series approaches a finite number as its terms are accumulated indefinitely. For
In calculus, infinite series are vital because they often represent complex functions in a simplified manner via power and Taylor series. To analyze these series, tests like the Ratio Test are applied. The convergence of an infinite series tells us whether the series approaches a finite number as its terms are accumulated indefinitely. For
- A finite sum means the series is convergent.
- An ever-growing sum means the series is divergent.